1. Field of the Invention
The present invention relates to lithographic apparatus and methods.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In such a lithographic apparatus, accurate positioning of an object or an object table is desirable. This object can, as an example, be an object table or an optical element such as a lens or a mirror. Such positioning can be realized by using an electromagnetic actuator. In general, an electromagnetic actuator includes two assemblies, a first assembly (also referred to as the magnet assembly) including at least one or more permanent magnets and a second assembly (also referred to as the coil assembly) including at least one or more coils. One of the properties of some electromagnetic actuators is that the force generated between both assemblies of the actuator is substantially independent of the relative position of both parts in the operating range of the actuator. This allows for a good dynamic response of the actuator since the relationship between current in the coil and generated force is known, regardless of the relative position of both parts. As a consequence, vibrations of one part are not transmitted to the other part as vibrations of one part do not cause a force variation on the other part. Assuming the coil assembly is mounted on a machine frame and the magnet assembly is connected to the object that requires accurate positioning or displacement, vibrations of the machine frame will not be transmitted to the object in case the force generated between both parts is substantially independent of the relative position of both parts. A number of state of the art actuators such as voice coil motors or linear actuators do, to some extent, have this property of providing a force that is substantially independent of the relative position of both actuator parts.
To avoid thermal disturbances of the object, it is the magnet assembly of the actuator may be attached to the object and not the coil assembly. Another reason for attaching the magnet assembly to the object and not the coil assembly is that the wiring of the coil assembly (from the coils to the amplifier) may cause disturbances during displacement of the coil assembly when it is attached to the object. Further the movement of wires and possibly water cooling tubes could potentially cause life cycle problems, since they are dynamically stressed during the life of the actuator.
The aforementioned actuators such as voice coil actuators or conventional linear actuators may, in some instances, employ a magnet having a weight that is significant and that may have some impact on dynamic response.
With such conventional actuators, a large part of the actuator coils is enclosed by the magnet assembly. Therefore, an effective cooling of the coils may be difficult.